Reconsidering CNG as an Alt-Fuel Solution

April 20, 2020

Stuart Birch

Mahle Powertrain’s turbocharged Di3 concept engine technology demonstrator is diagramed with the modifications needed to achieve a purely CNG-burning ICE. (Mahle)

As CO₂ emission standards continue to tighten, the future of the IC engine may be inexorably tied to alternative fuels. As automakers seek propulsion solutions delivering convenience and performance similar to gasoline or diesel, without radical changes to infrastructure, some experts say it’s time to re-consider the gaseous-fuel alternative.

Chief Engineer for Research Mike Bassett says an engine designed for purely CNG operation needs to have the robustness of a diesel but with the much higher rev limit of a gasoline unit. (Mahle)

OEMs and markets have largely rejected CNG (compressed natural gas) and LNG (liquified natural gas), preferring hybridization as a steppingstone to full electrification. Those fuels did not enter mainstream use “because their benefits have been compromised by dual-fuel installations that require the engine to also operate on gasoline,” argues Mike Bassett, chief engineer for Research at Mahle Powertrain.

To unbridle CNG’s full potential, Bassett’s team has developed a demonstrator vehicle running on CNG exclusively. Test data shows a 50% reduction in CO2 emissions at peak power and 20-40% in the RDE (real driving emissions) range. The combustion chemistry of an ultra-short carbon chain directly reduces the CO₂ produced by CNG, and its higher knock resistance rating (over 120 RON) permits very high compression ratios, above 13:1 on Mahle’s turbocharged Di3 direct-injected, 3-cylinder concept engine.

CNG it is also better able to meet the requirements of today’s more demanding emissions test protocols, including RDE, Bassett said. “The higher load cases used in RDE testing expose inefficient gasoline engine practices such as over-fueling to reduce exhaust temperatures; CNG doesn’t resort to this measure,” he explained. Particulate emissions that require costly aftertreatment on both diesel and DI gasoline engines are largely eliminated, he said, because a gaseous fuel mixes more homogeneously with intake air.

Mahle Powertrain’s fuel properties’ comparison list. (Mahle)

An engine designed specifically for CNG operation requires the robustness typically found in a diesel, combined with the rev limit of a gasoline engine because higher cylinder pressures (180 bar/2611 psi on the Mahle Di3) can be generated operating on natural gas than gasoline. And combustion speed does not limit engine revs in the way it does for a diesel, Bassett explained. Detail design changes would be required to the CNG engine’s pistons, piston pins, connecting rods and bearing shells to accommodate higher loads, and alternative valve and seat insert materials to cope with reduced lubricity.

Current hardware for direct injection must be specifically optimized for CNG operation, all this underlining why dual fuel engines are compromised. Compared to the infrastructure investment required to switch the private vehicle parc over to electric propulsion, the change to CNG as an interim fuel solution would be “relatively straightforward” in countries with an existing national gas distribution network, Bassett noted.